In a conventional inspection, defects in a pattern formed on a mask to be used for photolithography can be identified only after transferred onto a wafer. In recent years, in order to enable defects in a pattern to be detected in advance, a defect-transferability consideration inspection considering a transfer condition of defects onto a wafer has been adopted in an inspection on defects in a pattern.
In the defect-transferability consideration inspection, a first inspection is first performed to compare an optical image obtained by imaging a mask to a reference image generated based on design data of the mask and detect a portion in which a difference between the optical image and the reference image is equal to or larger than a predetermined threshold for defect determination as a defect. In order to increase the defect detection accuracy, the threshold is set to a low value in the first inspection to purposely detect an excessive number of defects. Following the first inspection, in the defect-transferability consideration inspection, a second inspection is performed to detect true defects from the defects detected in the first inspection in accordance with a predetermined algorithm considering a transfer condition of defects onto a wafer.
In the first inspection, a large quantity of defects as many as a million defects are sometimes detected on the entire face of a mask. Many true defects may be included in the defects detected in the first inspection. However, in a D-DB (Die to Database) inspection, when a reference image is generated based on design data of a mask, an abnormality may occur in the reference image due to an error of an inspection system, such as garbled references, in a process of generating the reference image. Due to occurrence of an abnormality in the reference image, many defects caused by the abnormality in the reference image may occur in the defects detected in the first inspection. Furthermore, as the defects caused by an error of the inspection system, many defects due to misalignment of the mask, light amount variation of a light source for detecting an optical image of the mask, and the like may occur, in addition to the defects due to the abnormality in the reference image. Because these defects caused by an error of the inspection system, that is, pseudo defects are not true defects, it is required to prevent these defects from being erroneously detected as defects.
In inspections before using the defect-transferability consideration inspection, a relatively small number of defects, such as several to several tens of defects are detected on the entire face of a mask. Therefore, defects caused by an error of an inspection system can be detected based on a fact that an excessive number of defects are generated and then an error stop of the inspection can be performed.